Facts and advice on acoustic control

Get it right from the start

In order to improve the process and achieve the best result, sound reduction requirements should be considered and discussed at the beginning of a project. In the UK residential market an acoustician is generally appointed to assess the noise level on site and advise on the best acoustic strategy. Here we have collected the most important points to take in consideration.

Indexes and terminology

UK noise policy

The World Health Organization (WHO) defines any noise above 65 decibels as noise pollution. In residential developments, the BS8233:2014 states that the noise in each habitable room should not exceed 35 dB and indicates that restful sleep is possible only if ambient noise levels are below 30 dB. If for example loud traffic noise registers 65-70Db, then the building must be able to reduce the noise level by 35-40Db.

It’s important to understand that external noise in residential projects can impact each housing block differently, so the acoustic assessment should be carried out for each elevation, to make sure that the correct sound reduction strategy is implemented across the whole project.

Sound reduction indexes



The sound reduction index (R) is expressed in decibels (dB) and is used only to describe the index for a single component or partition. Put simply, the index expresses the difference between the sound intensity hitting one side of a component or partition and the resulting sound measured on its other side.

The formula for deriving the sound reduction index of a single component or partition is as follows:

Sound reduction Index (R) = L1 – L2 + 10lg (S/A) dB

L1: average sound pressure level in the source room

L2: average sound pressure level in the receiving room

S: area of the test space (m²)

A: the equivalent sound absorption area of the receiving space

It’s important to remember there are 2 type of R index:

  • R: Reduction figure measured in laboratory [dB]
  • R': Reduction figure measured in the building [dB]


The ‘weighted sound reduction index.’ It is a number used to rate the effectiveness of the glass as a noise insulator and is measured in decibels (dB). It also takes into consideration a correction factor for the response of the human ear. For example, if a busy street has a noise level of 75dB and the required level of noise in a room is 40dB, the Rw value of the required window is 35dB.

It is worth remembering that every 10dB reduction is perceived by the ear as being half as loud, so that a 20dB noise reduction would reduce the perceived noise by 75%.


A is a single number value that is a measure of the sound insulation against commonly occurring noise in buildings (high-frequency sounds such as speech, music, noise from high-speed rail and road traffic)


Single number value that is a measure of the sound insulation against low frequency sounds (Rhythmic music with strong bass, noise from city traffic)

Lday, Levening, Lnight

Average sound pressure levels for day, evening and night on all days of a meteorological reference year. The sound pressure level depends on the volume of the room, surfaces and reverberation time [dB]


Weighted reduction figure in dB for ventilation opening


Average weighted noise level during the day, evening and night period with increased weighting in the evening and night period.

Starting a project with sound requirements

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Clarify how sound reduction index are given

There can be several dB difference between the three sound reduction figures Rw, Rw+C and Rw+Ctr for the same window.

Rw is the ‘weighted sound reduction index.’ It is a number used to rate the effectiveness of the glass as a noise insulator and is measured in decibels (dB).

The spectrum adaption terms C and Ctr are used to take into account different source spectra.

Ctr for example is an adjustment factor which is used to account for low frequency noise - typically the biggest problem with sound insulation. Ctr is always a negative number, so the Rw+Ctr will always be less than the Rw value. Many sound insulation types will represent how effective they are by displaying the Rw/Rw+Ctr values together.

It is therefore important to clarify whether the requirements for reduction figures are stated in Rw, Rw+C or Rw+Ctr.





Understand the important role of the acoustic consultant

In the UK residential market an acoustician is generally appointed to assess the noise level on site and advice on the best strategy to achieve the sound reduction target. Detailed information on requirements and façade noise levels are vital for VELFAC, as these impact on the windows configuration (materials, type of glass and window size) and ultimately on the price.





Consider sound requirements in your tender package

Specify your project’s sound reduction targets (Rw, Rw+Ctr or Rw+C) in your tender package. The sound-reducing effect of the window depends on the glazing construction and the window size, sash, frame and joint sealing. Therefore, it is important that tests are conducted of the entire window element and not only the glass.

Also be aware that the higher the sound reduction requirements are, the more limitation there will be in terms of windows size and opening functions.



Get a noise-reducing solution tailored to your project.


At VELFAC, we can help you achieve your project-specific acoustic requirements. Contact us as early as possible in your design process so we can provide test evidence and advise on the best solution for your project.


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We can provide test documentation on the noise reduction performance of our window solutions


"When we advise on a specific project, we take into account all the relevant factors such as sound pressure on the façade, room size, reverberation time and the total window area. All these elements are supported by test documentation, so you can be sure the products you specify match the requirements of your project."


- Neil Edwards, Specification Consultant, VELFAC


More information

VELFAC 200 Energy acoustic solutions

Visit our technical product database to learn more about acoustic performance and test results for our VELFAC 200 Energy system.



"We knew that VELFAC could meet our challenging performance and design criteria. Excellent acoustic control was a priority, as the site is surrounded by transport routes, including a railway line. We specified a mix of double and triple glazing across the build, and also installed both standard VELFAC vents (concealed within the slim frame) and acoustic trickle vents." "


- David Lomax, Senior Associate at Waugh Thistleton Architects


Get it right from the start. Talk to us about noise reduction

Contact a VELFAC expert early on in your design process, so we can help you achieve optimal noise reduction.